In numerous applications, it is necessary to apply a water-repellent and/or oil-repellent coating on surfaces exposed to liquids. In the case of ink-jet printheads, for example, it is necessary to apply an anti-wetting coating (AWC) on the printhead nozzle plate to prevent formation of residue of ink during and after ink-jet printing. In fact, the accumulation of residue around the orifice of the nozzle from which the drops of ink are expelled may cause an alteration of the direction of the drop, thus causing degradation of the quality of the printed images.
The anti-wetting treatment must further be applied only outside the orifice of the nozzles in order to prevent the printing resolution from being affected and must be chemically stable if it is put in contact with acidic or basic solutions, like many water-based inks, which would destroy the AWC in a short time.
Anti-wetting treatment of surfaces such as silicon, glass, or other inorganic or organic substrates may be obtained by depositing an anti-wetting polymeric layer by lamination, spin coating, or chemical vapor deposition (CVD).
The above treatments may offer good surface properties and excellent chemical stability, but are frequently unstable and may undergo delamination from the substrate when they are put in contact with the liquids. This phenomenon is due to the weak interaction of a physical type that binds together the deposited layer and the substrate. These physical interactions are in general due to hydrogen bonds or Van der Waals forces. Furthermore, the deposition techniques may cause application of the anti-wetting coating also within the orifice of the nozzle, thus causing alteration of the printing process.
Alternatively, an anti-wetting treatment may be obtained through a coating of a chemical type by creation of chemical bonds, which are stronger than physical bonds. Typically, this coating is obtained using molecules such as alkyl silanes or perfluoroalkyl silanes, chlorosilanes, or alkoxy silanes.
On the silicon surfaces, for example, alkyl silanes form a uniform monolayer (with a thickness ranging from a few angstrom to hundreds of nanometers) chemically bound to the silicon surface through an Si—O—Si bond.
The above coatings are not subject to delamination and enable the desired surface properties to be obtained through an appropriate choice of the alkyl tail. This type of coating is, however, known to be unstable when exposed to aqueous environments, as is the case of many water-based inks. In particular, the Si—O—Si anchoring bonds are unstable in aqueous environments, above all, if the pH is not neutral.